import re
scan = re.compile(flags=re.VERBOSE, pattern=r"""
(?P<number> \d+\.?\d* ) |
(?P<operator> \+ | - | \* | / ) |
(?P<eof> $ ) |
(?P<error> \S ) # catch all (except whitespace)
""").finditer
def parse(source):
parser = RPNParser()
for each in scan(source):
dispatch = getattr(parser, each.lastgroup)
dispatch(each.group())
return parser.stack[0]
class RPNParser(object):
def __init__(self):
self.stack = []
def number(self, token):
self.stack.append(token)
def operator(self, token):
right = self.stack.pop()
left = self.stack.pop()
self.stack.append(Expression(left, token, right))
def eof(self, token):
if len(self.stack) != 1:
raise Exception("source kaputt!")
def error(self, token):
raise Exception("unknown token %s!" % token)
class Expression(object):
def __init__(self, *args):
self.args = args
def __str__(self):
return "(%s %s %s)" % self.args
if __name__ == "__main__":
print parse("1.5 2 + 3.33 1.11 - *")
